Sperm stored in liquid nitrogen at -195.8 degrees C can remain viable indefinitely, provided the cold chain is maintained without interruption. The biological rationale is the same as for embryo storage – at temperatures below approximately -130 degrees C, all molecular motion ceases and biological degradation cannot occur. Under the Health and Care Act 2022, sperm can now be stored in the UK for up to 55 years with consent renewed every ten years.
What the long-term storage evidence shows
Successful pregnancies have been achieved from sperm stored for over 40 years in liquid nitrogen. The limiting factor on long-term sperm viability in storage is not biology but the reliability of the cold chain and the quality of the cryopreservation process at the point of freezing. Sperm that was poorly prepared or insufficiently protected by cryoprotectant before freezing will not improve with longer storage – the quality ceiling was set at the time of cryopreservation.
What affects post-thaw viability
Post-thaw motility and vitality are determined by several factors. The quality of the pre-freeze sample is the most significant – a sample with low motility and abnormal morphology before freezing will not produce high post-thaw motility. The cryoprotectant used, the freezing protocol, the quality of the controlled rate freezer used for slow freezing, and the warming technique all contribute. Post-thaw motility for a well-prepared donor sperm sample using a validated slow freezing protocol with the CBS DigitCool controlled rate freezer typically falls in the range of 30 to 50 percent of pre-freeze progressive motility.
Post-thaw viability is a measure of the entire cryopreservation system – the sample quality, the preparation, the cryoprotectant, the freeze protocol and the thaw technique. Any weak point in that chain limits what the final result can be, regardless of how long the sample has been in storage.
Storage consumables
Sperm samples in UK andrology laboratories are typically stored in CBS High Security Straws in 0.3ml or 0.5ml formats, held in visotubes and cryocanes within a liquid nitrogen storage vessel. For storage and consumable advice, contact Cryolab or use CryoGPT.
